Weather Window | La Nina Winters: A world of contrast

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TAHOE/TRUCKEE, Calif. - The agonizingly slow-starting winter of 2012 might seem like a distant yet painful memory by now, but in fact the Sierra Nevada water year for 2012 ended just last week on Sept. 30. Despite a disastrous first half of last season that combined record warmth with unprecedented dryness, last season might surprise you water-wise. The Central Sierra Snow Laboratory near Donner Pass tallied 45 inches of precipitation last year compared with an annual average of 51.5 inches. (Precipitation is rain plus snow melted for its water content.)

Randall Osterhuber manages the Snow Lab and last winter he measured more than 27 feet of snow-compared to an average of about 34 feet a year. However, those 325 inches of accumulated snowfall placed 2012 near the bottom of the pack, with a ranking of 50th snowiest winter since 1946. Snowfall for winter 2012 ranks along with similarly lackluster seasons such as 1990, 1991 and 1996. Taking the aggregate of eight weather stations from Highway 50 north to Mt. Shasta, overall precipitation in the Northern Sierra averaged 83 percent in 2012.

Not bad for a winter that featured the second driest December since 1920, and then set a record for consecutive dry days until a measly one-inch snowfall on Jan. 16 fell at Squaw Valley to break an historic streak of winter days with no precipitation. The 1 inch of snow was the first measurable powder to fall in more than a month at the resort, a meager amount that did nothing to improve ski conditions. By the middle of January Squaw Valley's upper mountain had picked up a grand total of 26 inches of snow - and that's including 8 inches that fell in early October. At the Reno-Tahoe Airport, the less than half inch of white stuff that coated the tarmac in mid-January was the first precipitation since Nov. 20, 2011, and it ended that location's longest winter dry spell in recorded history at 56 days.

Finally in late February, back-to-back storms from the Gulf of Alaska bombarded Tahoe resorts with up to 6 feet of fresh powder generating the winter's best skiing conditions to date. The revitalized storm track continued to hammer Tahoe during March as snowfall at regional resorts piled up. By March 30, Squaw Valley's upper mountain had added another 14 feet and a successful Easter vacation period was guaranteed. But for many businesses, all the snow came too late to make up for the dismal skiing conditions that plagued Tahoe resorts during the vital Christmas-New Year's holidays.

The recent back-to-back La Nina-influenced winters provided a remarkable contrast in weather patterns. While winter 2012 could get no traction storm-wise until the end of February, winter 2011 fired up early with more than 9 feet of snow before Thanksgiving. Winter 2011 dumped nearly 54 feet of snow on the Snow Lab, which ranks it as the fourth snowiest since 1946 and the ninth snowiest since 1878. The stark difference between the two winters offers a cautionary tale about using ENSO (El Nino Southern Oscillation) events in the tropical Pacific Ocean to predict winter storm patterns in the Sierra.

Well, the negative ENSO phase (La Nina) is history and the Climate Prediction Center has issued an El Nino Watch for winter 2013. Computer models are indicating "borderline ENSO-neutral/weak El Nino conditions" with possible strengthening during the winter months. But the warming trend in equatorial sea surface temperatures has slowed in recent weeks and the cooling water is leading forecasters to speculate whether this weak amplitude El Nino will actually verify. Compared with other El Nino events that started from La Nina conditions (1987, 1998, 2007, and 2010), the current sea surface temperatures are less elevated and indicate neutral conditions at this time.

It might be impossible to accurately predict this winter's weather, but we could definitely use some moisture. On Labor Day weekend the U.S. Department of Agriculture designated the state of Nevada and all counties along the eastern border of California as natural disaster areas due to severe or extreme drought conditions. That designation followed the warmest August on record in Reno, Nev. September 2012 also went down as the toastiest there since 1888, with an average temperature more than 5 degrees above normal. Sacramento set a new September record with 26 days at or exceeding 90 degrees.

The summer-like conditions in the lower elevations translated into one of the most pleasant Septembers at Lake Tahoe in memory, with high temperatures near 80 and lows in the 40s. The warm spell extended summer fun in the mountains into October.

Ironically and testament to California's diverse range of microclimates, San Francisco was extraordinarily chilly at more than 6 degrees below normal in September, setting a new record for the lowest monthly mean temperature at 58.1 degrees. The average daily maximum temperature in the City by the Bay was only 64 degrees, definitely not beach weather in what is normally a mild month on the coast.